Guaranteeing Safety of Learned Perception Modules via Measurement-Robust Control Barrier Functions

System observed by  $\mathbf p(\mathbf x) = \mathbf y$, state estimated by $\widehat{ \mathbf q}(\mathbf y) = \widehat{\mathbf x}$

Subset of state space:

$\mathcal C = \{\mathbf x \mid h(\mathbf x) \geq 0\}$

Safety defined as invariance of $\mathcal C$

System observed by  $\mathbf p(\mathbf x) = \mathbf y$, state estimated by $\widehat{ \mathbf q}(\mathbf y) = \widehat{\mathbf x}$

$\widehat{\mathbf q}$

This condition ensures the existence of $\mathbf u$ such that

$\frac{\partial h}{\partial \mathbf x}( \mathbf x)  \dot{ \mathbf x}  \geq -\alpha(h(\mathbf x))$

If $\mathbf u$ chosen such that $\dot{h}(\mathbf x, \mathbf u) \geq -\alpha( h(\mathbf x))$ then $h(\mathbf x)$ remains nonnegative, so the safe set $\mathcal C$ is invariant

$\dot h( \mathbf x, \mathbf u)  \geq -\alpha(h(\mathbf x))$

The set of inputs which ensure safety:

$K_\mathrm{cbf}(\mathbf x) = \{\mathbf u\mid \frac{\partial h}{\partial \mathbf x}( \mathbf x)  \mathbf f( \mathbf x) + \frac{\partial h}{\partial \mathbf x}( \mathbf x)\mathbf g(\mathbf x)\mathbf u  \geq -\alpha(h(\mathbf x))\}$

To filter a given controller $\mathbf k_d({\mathbf x})$ via convex optimization:

$\argmin\limits_{\mathbf u \in \mathbb{R}^m} \| \mathbf u - \mathbf k_d({\mathbf x}) \|$

$~~~~~~\text{s.t.} ~~L_{\mathbf f}h( \mathbf x) + L_{\mathbf g}h(\mathbf x)\mathbf u  \geq -\alpha(h(\mathbf x))$

Estimated state reconstructed by approximate inverse

$\widehat{\mathbf x} = \widehat{\mathbf q}(\mathbf y)=\mathbf x + (\widehat{\mathbf q}(\mathbf y) - {\mathbf q}(\mathbf y))=\mathbf x + {\mathbf e}(\mathbf y)$

Considering all $\mathbf x$ in uncertainty set:

$\min\limits_{\mathbf x\in \mathcal X(\mathbf y)} L_{\mathbf f} h(\mathbf x) + L_{\mathbf g}h(\mathbf x) \mathbf u + \alpha(h(\mathbf x)) \geq 0$

$\min\limits_{\|\mathbf e\|\leq \epsilon(\mathbf y)} L_{\mathbf f} h(\widehat{\mathbf x}-\mathbf e) + L_{\mathbf g}h(\widehat{\mathbf x}-\mathbf e) \mathbf u + \alpha(h(\widehat{\mathbf x}-\mathbf e)) \geq 0$

 (Clark 2020, Nilsson 2020)

We define the set of inputs which ensure robust safety:

$K_\mathrm{mr}(\mathbf y) = \{\mathbf u\mid L_{\mathbf f}h( \widehat{\mathbf x}) + L_{\mathbf g}h(\widehat{\mathbf x})\mathbf u - (a(\mathbf y) + b(\mathbf y)\|\mathbf u\|)  \geq -\alpha(h(\widehat{\mathbf x}))\}$

for given $a, b~:~\mathbb{R}^k\to\mathbb{R}_+$.

Main Result: As long as

• perception errors are bounded by $\epsilon(\mathbf y)$
• and $L_{\mathbf f}h$, $L_{\mathbf g}h$, and $\alpha\circ h$ are Lipschitz,

then by setting $a(\mathbf y) = (\mathcal L_{L_{\mathbf f}h}+\mathcal L_{\alpha \circ h})\epsilon(\mathbf y)$ and $b(\mathbf y) =\mathcal L_{L_{\mathbf g}h} \epsilon(\mathbf y)$,

any controller contained in $K_\mathrm{mr}(\mathbf y)$ renders the system safe.

The MR-CBF optimization problem (SOCP)

is feasible as long as

$\epsilon(\mathbf y) \leq \max \Big( \frac{\|L_{\mathbf g}h(\widehat{\mathbf x})\|}{\mathcal L_{L_{\mathbf g}h} },\frac{L_{\mathbf f}h( \widehat{\mathbf x}) +\alpha(h(\widehat{\mathbf x}))}{\mathcal L_{L_{\mathbf f}h}+\mathcal L_{\alpha \circ h}}\Big)$

which can be achieved from data

$\argmin\limits_{\mathbf u \in \mathbb{R}^m} \| \mathbf u - \mathbf k_d(\widehat{\mathbf x}) \|$

$~~~~~~~~\text{s.t.}~~L_{\mathbf f}h( \widehat{\mathbf x}) + L_{\mathbf g}h(\widehat{\mathbf x})\mathbf u$

$- ((\mathcal L_{L_{\mathbf f}h}+\mathcal L_{\alpha \circ h})\epsilon(\mathbf y) + \mathcal L_{L_{\mathbf g}h} \epsilon(\mathbf y)\|\mathbf u\|)  \geq -\alpha(h(\widehat{\mathbf x}))$

Segway robot constrained to planar motion

Safety defined by pitch angle and rate

$|\dot{\theta}_y +10(\theta_y-\theta_y^\mathrm{eq}) |\leq 4$

Pitch angle $\theta_y$ and position $r$ measured only by camera

Training data collected from grid of $\theta_y$ and $r$ ($N=800$)

Model mapping image to $\theta_y,r$ trained using sklearn kernel ridge regression with radial basis functions

CBF filter does not ensure safety, MR-OP filter does

For details, see our paper at https://arxiv.org/abs/2010.16001

Guaranteeing Safety of Learned Perception Modules via Measurement-Robust Control Barrier Functions

Sarah Dean    Andrew J. Taylor    Ryan K. Cosner    Benjamin Recht    Aaron D. Ames

References

1. Ames, Aaron D., Jessy W. Grizzle, and Paulo Tabuada. "Control barrier function based quadratic programs with application to adaptive cruise control." IEEE CDC, 2014.

2. Ames, Aaron D., et al. "Control barrier function based quadratic programs for safety critical systems." IEEE TAC, 2016.

3. Nilsson, Petter, and Aaron D. Ames. "Lyapunov-like conditions for tight exit probability bounds through comparison theorems for sdes." IEEE ACC, 2020.

4. Clark, Andrew. "Control barrier functions for complete and incomplete information stochastic systems." IEEE ACC, 2019.